1. Field of the Invention
The present invention relates to the suppression of signal interference in equipment, and more particularly to employing a new grounding technique in a power supply or adaptor to suppress signal interference in equipment due to variable ground current flowing through a ground loop.
2. Description of the Related Art
In computer, electrical and electronic systems, grounding is an important process. According to IEEE, It is defined as a conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth, or to some conducting body of relatively large extent in place of the earth. It""s used for establishing and maintaining the potential of the earth (or of the conducting body) or approximately that potential, on conductors connected to it, and for conducting ground current to and from the earth (or the conducting body). Besides, earthing is a process of connecting the grounding system to earth and is performed to keep the entire grounding system at earth potential.
A computer and electronic system, which includes a 3-pin power supply or adaptor, a personal computer (PC) or host unit, and a TV or some equipment, is shown in FIG. 1. The internal circuitry of the power supply 100 is simplified, for example, as a DC voltage source 102 at secondary side of the transformer in the power supply 100. xe2x80x9c3-pinxe2x80x9d is used to describe three conductors to the input of the power supply 100 for wire connections from Line, Neutral and Ground conductors of a power outlet. The three conductors include a line conductor 104, a neutral conductor 106, and a ground conductor 108. The frame ground 118 is the Ground conductor of the power outlet and is connected to earth. The PC 120 as the host unit of the system is simplified as a load resistor R1 and the power is supplied by the power supply 100. The TV 130 is coupled to the PC 120 for signal processing and control, and is also simplified as an input impedance R2 of the TV circuitry. Grounding is the process of interconnecting all of the power supply 100, the PC 120, and the TV 130, and then connecting them to the earth. R3, R4, R5 and R6 represent the total impedances of all cables between the power supply 100, the PC 120, the TV 130, and the frame ground 118. Grounding is achieved through the cables 114 connecting 108 to 118, 110 connecting 108 to 120, 112 connecting 120 to 130, and 116 connecting 130 to the frame ground 118. The DC voltage source 102 supplies a current I1 through a metal line to R1 of the PC 120. The current I1 flows through R1 and the line 110 carries the current I1 back to 108 to complete the voltage source-load circuit. Similarly, there are a line carrying signal to the input impedance R2 of the TV 130 and the line 112 carrying the signal back to the PC 120. Vout is the signal voltage of the PC 120 between the two lines that is supplied to the TV 130.
Any closed loop connecting 100, 120 and 130 serving as a signal reference loop is a ground loop (even though ground may not be involved). Practically speaking, there is impedance in all conductive lines, including any ground loop, so there is noise, interference voltages, between various points in any ground loop and in any conductive line when there is current flowing through them. The potential difference between any two points in a ground loop is often called ground voltage shift and is a major source of noise and interference in electronic circuits.
The current in a ground loop, called the ground current, may come from the voltage drop on R4 of the cable 110. The voltage causes a ground current Ig to flow through the loop. The ground current Ig would produce an interference voltage drop at the TV 130 side by flowing through R5, and thereby the voltage Vin of 130 across R2 would be about
Vin=Voutxe2x88x92Ig*R5
but not be equal to Vout of 120. Ig*R5 is the interference voltage drop at the TV 130 side. Ground currents, and thus ground voltage shifts, will cause distortion of signal at R2 of 130. Any potential in conducting lines connected to a ground loop with respect to a lower potential in the ground loop can cause ground currents.
Since the load current through R1 of the PC 120 is generally variable, the ground current Ig caused by a voltage across R4 is also variable. Therefore, the interference voltage (Ig*R5) is variable. The continuous change of the interference voltage is a significant noise and can interfere with the signals supplied to the TV 130 seriously, resulting in, for example, image quality degradations on the screen of the TV 130.
For the forgoing reasons, there is a need for suppressing the noise and signal interference problem due to ground current in a ground loop. A possible solution to the signal interference problem is using thick lines, for example No.18 AWG (American Wire Gauge) lines, as the signal line 112 in FIG. 1 to increase the protection of signal from noise and interference. Thick lines are not practical and suitable for computer and electronic systems. Another solution is reducing impedance of cable by reducing the lengths of conductors between 100 and 120 or 120 and 130 in the loop. This solution is also difficult to be implemented. As to grounding practice in a power supply, consider a typical power supply 200, shown in FIG. 2, it contains a transformer 202, a primary circuit on the primary side of the transformer 202 connecting to an external AC input voltage source VAC, and a secondary circuit on the secondary side of the transformer 202 producing the output voltage Vout of the power supply 200. The secondary circuit contains a diode D1 as a rectifier, a filter capacitor C and sometimes has an extra voltage regulator. The output voltage Vout is supplied to load equipment such as a PC. Between primary and secondary side, there is a capacitor C3 for reducing EMI level. Common grounding practice is to connect the secondary side 206 to the outside frame ground 210. In order to improve the signal interference problem, the present invention provides a better grounding method.
As described above, noise or signal interference problem has long been a serious concern in electronic circuits. The main object of the invention is to provide a new grounding technique in the design for an electronic power supply to suppress the signal interference problem in equipment of an electronic system including the power supply.
The traditional grounding practice is to connect the common signal references of the secondary circuits to the outside frame ground. On the contrary, the method of the present invention applied to the power supply is to form conductive paths connecting the common signal references of the primary and secondary circuits to the frame ground through a capacitor and an inductor connected to in series. The addition of a capacitor is to cut the otherwise direct conductive paths being parts of ground loops between the signal references and the ground conductors. The added capacitor is to reject DC or low frequency noise and helps keep electromagnetic interference (EMI) reduction performance because C1 and C2 can be adjusted as C3 of FIG. 2. The added inductor is to reject high frequency noise and interference, together with C2 behaving like a LC filter. Most noise and interference signals cannot pass through the formed conductive paths containing a capacitor and an inductor connected in series. But, the DC impedance of the inductor can""t be over 0.1 ohm in order to obey safety regulations. As a result of this grounding technique, the degree of signal interference problem is greatly reduced. Accordingly, the invention provides a good method for suppressing the inevitable signal interference problem in electronic circuits.